1. Field of the Invention
The present invention relates to a liquid ejection head having a plurality of nozzle arrays.
2. Description of the Related Art
A recording device such as a printer, a copy machine, and a facsimile is configured to record an image of a dot pattern on a recording medium such as a paper sheet and a plastic thin plate based on image information. The recording method of the recording device can be classified into an ink jet method, a wire dot method, a thermal method, a laser beam method, and the like. Among them, the recording device that uses the ink jet method (ink jet recording device) ejects and flies ink droplets (liquid) from ejection orifices of nozzles of a recording head and attaches the ink droplets to a recording medium to perform recording.
In recent years, high-speed recording, high resolution, high image quality, low noise, and the like are required for recording devices, and the ink jet recording device is one of the recording devices that satisfy such requirements.
A configuration of a liquid ejection head used in a recording device that ejects liquid such as ink in the manner as described above will be described below. The liquid ejection head includes an element substrate provided with energy generating elements, for example, electrothermal transducers for generating energy for ejecting liquid and a flow passage forming member (also referred to as “orifice substrate”) that is bonded to the element substrate and forms liquid supply paths (passages). The flow passage forming member has a plurality of nozzles in which liquid flows, and an opening at the top end of the nozzle forms an ejection orifice for ejecting liquid droplets. The nozzle has a bubbling chamber in which bubbles are generated by the energy generating element and a passage for supplying liquid to the bubbling chamber. An electrothermal transducer is disposed in the bubbling chamber in the element substrate. A supply port is provided in a main surface of the element substrate which is in contact with the flow passage forming member, and a back surface supply port is provided in the back surface opposite to the main surface. A common liquid chamber is provided between the supply port and the back surface supply port. In the flow passage forming member, ejection orifices are provided at positions facing the electrothermal transducers on the element substrate.
In the recording head configured as described above, liquid supplied from the back surface supply port to the common liquid chamber is supplied to each nozzle through the supply port and filled in the bubbling chamber. The liquid filled in the bubbling chamber is flown in a direction approximately perpendicular to the main surface of the element substrate by the bubbles generated when the liquid is film-boiled by the electrothermal transducer, and ejected from the ejection orifice as a liquid droplet.
To achieve a higher resolution recording image by the liquid ejection head, it is desired to reduce the size of the liquid droplet and reduce the dot diameter formed on a recording medium. However, if the size of the liquid droplet is reduced, the throughput decreases unless the number of liquid droplets ejected to a recording medium such as paper per unit time is increased. Therefore, as a method for increasing the number of liquid droplets ejected per unit time, it is considered to increase the number of the nozzles.
In recent years, to achieve recording of higher resolution image at higher speed, liquid ejection head having wider printing width and higher density of nozzle arrangement is required. Hereinafter, a conventional example of a liquid ejection head corresponding to the requirements and the recording method thereof will be described.
In this liquid ejection head, heaters are provided on a silicon substrate as energy generating elements, and nozzles are formed by nozzle members. Liquid is supplied from the back surface of the silicon substrate through a liquid supply port formed as a hole penetrating the silicon substrate. Electric energy is applied to the heater to heat and bubble the liquid, and thereby the liquid is ejected from the ejection orifice to perform recording on a recording medium. The electric energy is applied to the heater by a driving transistor provided on the silicon substrate through an electric circuit substrate and a flexible circuit substrate according to a signal inputted from outside via an electric connector. Methods for forming high density and high accuracy nozzles and ejection orifices in such a liquid ejection head are disclosed in Japanese Patent Laid-Open No. 05-330066.
To perform high-speed printing (recording of image) by using such a liquid ejection head, a method is known in which a large number of liquid ejection orifices are arranged over the entire width of the recording medium. In this case, it is possible to record all print data (image data) while scanning the recording medium once with respect to the liquid ejection head (one-pass drawing method using a full multi-head). In such a liquid ejection head, if there is only one defective nozzle among a large number of nozzles, defective printing occurs. Therefore, a method is proposed in which, even if there is a defective nozzle, defective printing is complemented by using the other nozzles. Such a method will be described with reference to FIG. 7. In FIG. 7, each square box 501 indicates a pixel on the recording medium 500 and each black dot 502 indicates the ejected liquid.
FIG. 7 shows an example of a conventionally known method for improving defective printing when there are some defective nozzles. In FIG. 7, the nozzle array of the liquid ejection head is arranged along the X direction, and the liquid ejection head performs printing while scanning the recording medium 500 in the Y direction. Although the liquid ejection head should form a printing pattern as shown in FIG. 7A, a white streak is generated as shown in FIG. 7B if there is a nozzle that cannot eject liquid for some reason. To improve this, as shown in FIG. 7C, complementary dots 503 are ejected to the positions adjacent to pixels to which the non-ejection nozzle should eject liquid by using nozzles adjacent to the non-ejection nozzle.
Further, as another example of complementing the non-ejection nozzle, in U.S. Pat. No. 5,984,455A, a primary nozzle and a secondary nozzle arranged along the scanning direction are disclosed. If a defect is detected in either the primary nozzle or the secondary nozzle, in place of a pressure generating element (energy generating element) of the defective nozzle, a pressure generating element of the other nozzle is operated. In this way, data (pixels) that should be formed by the defective nozzle are formed by the other nozzle located on the same axis in the scanning direction as that of the defective nozzle.
If there are a plurality of nozzles on the same axis in the scanning direction, not only it is possible to complement the non-ejection nozzle and improve throughput, but also there is an advantage that liquid droplets ejected from a plurality of different nozzles can be provided to the same pixel array on the recording medium. Thereby, a high resolution image quality that seems as if it were drawn by multiple passes can be obtained. This will be described with reference to FIG. 8. FIG. 8A shows a situation in which an image is formed on the recording medium 500 by a liquid ejection head having only one nozzle array L1 and having only a single nozzle on the same scanning axis (axis along the scanning direction Y). In FIG. 8, the dots denoted by reference numeral 502 indicate liquid droplets landed on the recording medium 500 (landed dots). If the nozzles in the nozzle array L1 include a nozzle n1 whose liquid droplet lands on a position shifted from an ideal landing position for some reason, a streak 5 is formed along the scanning direction Y in the recording image (see FIG. 8A). On the other hand, FIG. 8B shows a situation in which an image is formed on the recording medium 500 by a liquid ejection head having four nozzle arrays L1 to L4 and including four different nozzles on the same axis along the scanning direction Y. In this case, an influence to an image caused by one defective nozzle n1 can be suppressed by the other three normal nozzles n2 to n4. Specifically, the liquid droplet 505 from the nozzle n1 is formed every four dots, so the influence thereof is difficult to recognize. As a result, a higher resolution image can be obtained in the configuration including a plurality of nozzle arrays shown in FIG. 8B than in the configuration including a single nozzle array shown in FIG. 8A.
There is a method for increasing recording density in the nozzle array direction by reducing the amount of liquid droplet to be ejected in order to obtain high resolution image. Therefore, it is known that, in each nozzle array, nozzles are arranged in a zigzag pattern instead of simply and linearly arranging the nozzles. Specifically, a zigzag shaped nozzle array is formed by alternately arranging a nozzle located far from the common liquid chamber (hereinafter also referred to as “long nozzle”) and a nozzle located near the common liquid chamber (hereinafter also referred to as “short nozzle”). Such a zigzag shaped nozzle array improves density of the nozzle arrangement compared with a linear nozzle array, so recording density of an image can be improved.
To obtain high resolution image, it is desired that the long nozzles and the short nozzles arranged alternately have substantially the same ejection characteristics such as the amount of ejection and the speed of ejection. However, a difference of ejection characteristics may occur between the long nozzle and the short nozzle due to manufacturing tolerance, driving condition, and operating environment. Because of this, density unevenness and landing error occur between a pixel array on a recording medium formed by using only the long nozzle and a pixel array formed by using only the short nozzle, and a good image may not be obtained.
Further, the position and the shape of a dot formed by a liquid droplet landed on a recording medium are varied depending on the orientation of the nozzle from the common liquid chamber, and the difference of the orientations of the nozzles may affect the image quality. This will be described with reference to FIG. 9. As shown in FIGS. 9B and 9C, when nozzle arrays LL and LR are arranged on both sides of the common liquid chamber 912 having a slit-like opening in the substrate 910, the orientations Dnl and Dnr of the passages connected from the common liquid chamber 912 to the nozzles Nnl and Nnr are opposite to each other for the nozzle arrays LL and LR. In other words, the nozzle arrays LL and LR are designed to be line symmetric to each other with respect to the slit-like opening of the common liquid chamber 912 that is used as the central axis. In the example shown in FIG. 9C, the nozzle arrays LL and LR are formed by nozzles that are linearly arranged.
Between the pair of nozzle arrays provided on both sides of the common liquid chamber 912, the shape of the nozzle (position of the opening and shapes of the passage and the ejection orifice) may be shifted or deformed in the manufacturing process, or changes over time in the ejection characteristics may occur during use in each nozzle array. Therefore, a difference of characteristics such as the speed of ejection and the amount of ejection may occur between the nozzle arrays LL and LR.
In addition, the shape of a dot landed on the recording medium may vary depending on the nozzle array. In each nozzle of the liquid ejection head, it is known that a liquid droplet ejected by one ejection operation is divided into a main droplet 901a or 901b and a satellite droplet 902a or 902b smaller than the main droplet (see FIG. 9B). The flying speed and the ejection angle of the main droplet 901a or 901b and the satellite droplet 902a or 902b are different from each other, so the two types of droplets ejected while the nozzles are scanning the recording medium are landed at different positions on the recording medium. If the dots formed by the satellite droplets 902a and 902b are too distinct, the dots can be viewed at positions irrelevant to the image data, so the dots causes degradation of the image. The degree of the shift of landing position of the main droplets 901a and 902b and the satellite droplets 902a and 902b may vary depending on the orientations of the passages 916l and 916r from the common liquid chamber 912 to each nozzle Nnl and Nnr. This is shown by FIG. 9A. The satellite droplets 902a and 902b are easily affected by the orientations of the passages 916l and 916r in the forming process of the droplets ejected from the nozzles Nnl and Nnr, and may be flown at an ejection angle different from that of the main droplets 901a and 901b. Thereby, the shift between the landing positions, which are formed on the recording medium, of the main droplet 901b and the satellite droplet 902b ejected from the nozzle array LL may be different from the shift between the landing positions, which are formed on the recording medium, of the main droplet 901a and the satellite droplet 902a ejected from the nozzle array LR. Therefore, if pixel arrays are formed by using one nozzle array only, density unevenness and streaks may occur between the pixel arrays and pixel arrays formed by using the other nozzle array only. Thus, a good image may not be obtained.
As described above, if there are nozzles whose passages have different lengths or nozzles whose orientations from the common liquid chamber are different, a difference of ejection performances of liquid droplets ejected from the nozzles occurs, and as a result there is a problem that the quality of recording image degrades. In particular, in a zigzag shaped nozzle array in which nozzles are densely arranged, there is a problem that the recording image is affected by a difference of ejection characteristics caused by a difference of the length of the passage, a difference of ejection characteristics generated by a difference of the orientations of the passages from the common liquid chamber to each nozzle, and a difference of landing positions of the satellite droplets.
In particular, in the case of a line head which has nozzle arrays having a length corresponding to the recording width and performs recording by scanning the recording medium by the recording head only once, the degradation of the image quality due to the above problems appears remarkably.